[PhaseLockk]

Exactly this. It may be useful for OP to consider what the hypothetical opposite of significant digits would be: insignificant digits, ie. digits that tell you nothing. A leading zero can be omitted from a number without any loss of information. A trailing zero, when correctly used in a position that is not below the tolerance, gives you significant information. On the other hand, if you start combine numbers with different tolerances, but do not truncate the result, you will have a bunch of trailing digits that are meaningless in practice because they are below the combined tolerance.